The present invention relates to the field of engine control, and more particularly to a method and a device for controlling the first opening of an electronic thermostat regulating the temperature of an internal combustion engine, particularly of a vehicle.
Engine control is the technique of managing an internal combustion engine with all its sensors, actuators and inter-system links (ISL). All the monitoring and control laws (the software strategies) and characterization parameters (calibrations) of an engine are contained in a computer called an electronic control unit (ECU).
Internal combustion engines (with a carburetor or with fuel injection), require cooling that is generally obtained by circulating water or air. This is because an internal combustion engine generates heat. This heat is due to the combustion of the gases and to the various frictions of the moving parts.
In order to confine the expansion between permissible temperature ranges and to preserve the characteristics of the lubricating oil, the engine must therefore be cooled. This cooling is achieved by means of three fluids present in the environment:                Air is essentially used for the heat exchangers (radiator, air—air heat exchangers), and for the natural cooling of the heat generated by conduction on the outside of the engine and of the exhaust.        Water cools the cylinders and the cylinder head.        Oil removes the heat from the pistons, the camshaft, the big ends, the crankshaft bearings, the valves.        
When water is circulated, the water generally flows in a closed circuit and is cooled in a radiator, which is itself cooled by air whereof the flow may be assisted by a fan. Since the heat to be removed depends on the speed and load of the internal combustion engine and since the temperature thereof must be kept within a relatively narrow range to obtain optimal efficiency, the cooling system must be regulated. For this purpose, it is necessary to determine a regulation temperature with a setpoint to guarantee an optimal operating temperature for the engine.
One objective of this regulation is to decrease the fuel consumption and thereby obtain a commensurate decrease in the formation of pollutants such as nitrogen oxides.
In fact, at low temperature, the lubricating oil of the internal combustion engine has a high viscosity, which causes additional friction in the engine and consequently extra fuel consumption. This occurs in particular upon starting the vehicle when the internal combustion engine and the oil are cold.
The production of nitrogen oxides (NOX) depends in particular on the temperature of the gas mixture introduced into the cylinders of the vehicle internal combustion engine. The higher the mixture temperature, the greater the production of nitrogen oxides.
Thus by increasing the temperature of the engine cooling water, the oil temperature is thereby increased and the friction losses are reduced, whereas a lower temperature will limit the production of nitrogen oxide in particular. It is therefore important to maintain the engine at an optimal temperature. Hence the advantage of the cooling circuit. Thus, most vehicles today are equipped with a conventional thermostat operating on the principle of the expansion of a volume of wax. This thermostat may be placed at the outlet or inlet of the engine. Under the effect of the water temperature, a wax plug, through its thermal expansion calibrated against a temperature point, controls the opening of one or two check valves via a working piston. The one or two valves will open or close or regulate the coolant circulation, and optionally a bypass circuit.
Yet a problem arises particularly during the first starting of the vehicle. In fact, during the cold starting, the coolant circumvents the main radiator via a bypass to return directly into the internal combustion engine, entering the lower part thereof, that is in the engine block. When the coolant temperature rises, a valve opens a second coolant path passing through the main radiator, which cooperates with a fan to extract the surplus heat from the coolant.
Before the valve opens, the temperature difference on either side of the thermostat is amplified as the engine temperature rises. When the valve opens for the first time, a thermal shock is therefore produced. Once the coolant circulation is produced, the coolant does not fall back to the initial temperature. Even if the engine undergoes another thermal shock, the greater therefore occurs upon the first opening of the thermostat.
This thermal shock has negative effects on the auxiliary components surrounding the engine and, in particular, the cooling systems for the engine itself and other cooling systems (passenger compartment unit heater, EGR heat exchangers, water/oil heat exchangers, air conditioning radiator and its condenser, turbocharging air, BVA radiator, etc.) which are vulnerable to changes or passages from one temperature to another engine operating temperature setpoint, generating additional stresses thereon. Similarly, the position at the various elements with regard to each other has an influence on the quality of the temperature regulation and the associated stresses. For example, if the cooling circuit valve control system is located at the inlet or outlet of the engine.
These additional stresses cause deformations of the auxiliary components of the engine generated by temperature variations and result in additional mechanical forces.
Patent US 2002/0053325 proposes a solution to this problem with a device for regulating the engine temperature by controlling the coolant circulation between the engine and a heat exchanger such as a radiator. This device comprises a first conventional thermostat as described previously, of which the activation temperature is around 102° C., and a thermostat comprising a central resistor and of which the activation temperature is 25° C. higher than that of the first thermostat, and is therefore around 127° C. These two thermostats each control a different opening of a valve enabling the coolant to enter the radiator to be cooled. Thus when the coolant temperature reaches the first activation temperature, the expansible material contained in the first thermostat activates a piston which itself opens a valve thereby enabling the coolant to flow via the radiator at a certain flow rate. If the liquid temperature is higher and is about 127° C., the expansible material contained in the second thermostat activates a second piston which allows a wider opening of the valve. The liquid thus flows with a higher flow rate to the radiator, so that within the same time interval a larger quantity of liquid will be cooled. This device therefore serves to cool the liquid faster and thereby to prevent the engine from remaining at an excessively high temperature for too long.
However, one drawback of the device is that it only acts when the temperature of the engine is higher than an ideal operating temperature, which is about 90° C. (or according to the rating of the thermostat). It does not help to avoid the thermal shock. The various parts of the engine concerned by this overheating are therefore subject to the stresses and deformations mentioned above.